1. Field of the Invention
This invention relates to a pneumatic tire having a bead core comprised of a core wire and plural sheath layers laminated therearound.
2. Description of Related Art
In general, a large stress is applied to a bead portion in a pneumatic tire used under a high speed and a heavy load, e.g. a pneumatic tire for an airplane during the running of the tire. Therefore, a bead core durable to such a large stress should be arranged in the bead portion.
As the bead core durable to the large stress, there has hitherto been known a bead core as disclosed in JP-A-53-51804 or so-called cable bead formed by helically winding plural thin-diameter wires around a single thick-diameter wire to form a sheath layer and laminating a plurality of such sheath layers one upon the other. In such a bead core, each of the sheath layers is formed by winding the sheath wires at a close-packed state so as to bear the above large stress. The term xe2x80x9cclose-packed statexe2x80x9d used herein means a state that a maximum number of sheath wires are embedded in each of the sheath layers.
However, when the pneumatic tire having such a conventional bead core is used over a long time, there is a problem that fatigue breakage is caused in the sheath wires of a second sheath layer from an outermost side to lower bead portion durability as mentioned below. That is, since each of the sheath layers is constructed by winding the sheath wires at a close-packed state and the number of sheath wires embedded at the close-packed state is necessarily a positive integer, outer surfaces of adjoining sheath wires (bare wires) are not usually closed to each other, so that a slight gap is existent between the sheath wires (a sum of all gaps in one sheath layer is more than zero but is smaller than a diameter of one sheath wire). In the vulcanization of such a pneumatic tire, therefore, rubber disposed around the bead core flows into the inside of the bead core through a slight gap between adjoining sheath wires in a radial direction. For example, in case of the pneumatic tire for airplanes, rubber covers a full periphery of a sheath wire in an outermost sheath layer and flows inward into a second sheath layer from the outermost side in the radial direction through gaps between the sheath wires of the outermost sheath layer so as to cover about 10-20% of an outward portion of each sheath wire in the second sheath layer in the radial direction. For this end, in the conventional bead core, a greater part of adjoining sheath wires in the second sheath layer from the outermost side are opposite to each other at a bare state, while all adjoining sheath wires from a third and on sheath layers are opposite to each other at a bare state. At this state, when such a pneumatic tire is run at a high speed under a heavy load, a deformation quantity produced in each sheath layer (sheath wires) becomes larger as the sheath layer becomes near to the outermost side, so that the adjoining bared sheath wires in the second sheath layer directly contact with each other to strongly cause fretting and hence a large stress is repeatedly caused in the inside of the sheath wire in the second sheath layer to finally cause fatigue breakage. Although the deformation quantity produced in the outermost sheath layer is largest among the plural sheath layers, the full periphery of each sheath wire in the outermost sheath layer is covered with rubber as mentioned above, so that the adjoining sheath wires do not directly contact with each other and hence the fatigue breakage hardly occurs in the sheath wires of the outermost sheath layer.
It is, therefore, an object of the invention to provide a pneumatic tire capable of improving bead portion durability by effectively controlling the fatigue breakage of the sheath wire in the second sheath layer from the outermost side.
According to a first aspect of the invention, there is the provision of a pneumatic tire having a bead portion provided therein with a bead core formed by helically winding plural thin-diameter sheath wires around a thick-diameter core wire to form a sheath layer and laminating a plurality of such sheath layers one upon the other, wherein an end count of sheath wires in an outermost sheath layer is made smaller than an imaginary end count of sheath wires in the outermost sheath layer at a close-packed state.
According to a second aspect of the invention, there is the provision of a pneumatic tire having a bead portion provided therein with a bead core formed by helically winding plural thin-diameter sheath wires around a thick-diameter core wire to form a sheath layer and laminating a plurality of such sheath layers one upon the other, wherein a periphery of each sheath wire constituting a second sheath layer from an outermost side is previously covered with a coating rubber.
When an end count of sheath wires in an outermost sheath layer is made smaller than an imaginary end count of sheath wires in the outermost sheath layer at a close-packed state as defined in the first aspect of the invention, a sum of gaps existing between the sheath wires in the outermost sheath layer is wider only by product of reduced end count and diameter of sheath wire than a sum of gaps existing between sheath wires at the imaginary close-pack state (which is larger than zero but smaller than the diameter of the sheath wire as previously mentioned), and hence the gap between the adjoining sheath wires is widened. When the vulcanization of the pneumatic tire is conducted at such a state, rubber easily flows inward up to the periphery of the sheath wire in the second sheath layer through the widened gap between the adjoining sheath wires of the outermost sheath layer in the radial direction. For example, when one sheath wire is decreased from the end count at the imaginary close-packed state, rubber covers full periphery of each of the sheath wires in the second sheath layer and flows inward into a third sheath layer in the radial direction so as to partly cover an outward portion of a sheath wire in the third sheath layer in the radial direction. Thus, fretting due to direct contact between adjoining sheath wires in the second sheath layer is prevented and hence the fatigue breakage of such a sheath wire is effectively controlled.
Further, when the periphery of each sheath wire constituting the second sheath layer from the outermost side is previously covered with the coating rubber as defined in the second aspect of the invention, the direct contact between the adjoining sheath wires in the second sheath layer is suppressed by the coating rubber to prevent the fretting, and hence the fatigue breakage of such a sheath wire is effectively controlled.
In a preferable embodiment of the first aspect, the end count of the sheath wires in the outermost sheath layer is not less than 0.8 times the end count at the imaginary close-packed state, whereby the fatigue breakage of the sheath wire can effectively be controlled without lowering the strength of the bead core.
In another preferable embodiment of the first aspect, an end count of sheath wires in a sheath layer(s) existing between outermost sheath layer and innermost sheath layer is made smaller than an imaginary end count of sheath wires in this sheath layer at a close-packed state, whereby the fatigue breakage of the sheath wire in second and on sheath layers from the outermost side can be controlled.
In a preferable embodiment of the second aspect, the coating rubber has a thickness of from 0.2 mm to 0.2 times a diameter of the sheath wire, whereby the fatigue breakage of the sheath wire can be controlled without increasing weight.